Method of obtaining a filament-containing composite with a boron nitride coated matrix

ABSTRACT

A composite is produced by admixing a matrix-forming material with organic binding material, forming the resulting mixture into a tape, disposing a layer of spaced boron nitride coated filaments between at least two of the tapes to form a layered structure, laminating the layered structure, heating the layered structure to remove organic binding material and hot pressing the resulting porous structure to form a composite containing spaced boron nitride coated filaments.

This application is related to Ser. No. 056,516, filed Jun. 1, 1987, forBorom et al.; Ser. No. 066,271, filed Jun. 25, 1987 now abandoned infavor of Ser. No. 216,488, filed Jul. 8, 1988; Ser. No. 102,054, filedSept. 28, 1987 now abandoned in favor of Ser. No. 216,471, filed Jul. 8,1988; and Ser. No. 100,806, filed Sept. 24, 1987; all for Singh et al.The referenced applications are directed to the production offiber-containing ceramic composites. They are assigned to the assigneehereof and are incorporated herein by reference.

The following application is assigned to the assignee hereof andincorporated herein by reference:

U.S. application Ser. No. 132,753 filed on or about Dec. 14, 1987 forFilament-Containing Composite, R. N. Singh and A. R. Gaddipati,discloses admixing a matrix-forming material with organic bindingmaterial, forming the resulting mixture into a tape, disposing a layerof spaced filaments between the tapes to form a layered structure,laminating the layered structure, heating the layered structure toremove organic binding material and hot pressing the resulting porousstructure to form a composite containing spaced filaments.

The present invention is directed to producing a boron nitride coatedfilament-reinforced ceramic matrix composite.

Fiber reinforcement of brittle ceramic materials offers significantopportunities for toughening of the brittle matrix. For this reasonceramic matrices are being incorporated into fiber preforms for thefabrication of ceramic matrix composites. Several techniques forincorporating the ceramic matrix into a fiber preform have been tried.These are: filament-winding through a slurry of the matrix material,chemical vapor infiltration and sol-gel infiltration techniques. Inpassing a filament winding through a slurry of the matrix, relativelysmall amounts of the matrix adhere to the filaments. Chemical vaporinfiltration and sol-gel infiltration techniques are slow. Conventionalceramic processing techniques such as slip casting and/or vacuum castingtechniques followed by hot-pressing do not provide good penetration ofthe matrix material between the reinforcing fiber preforms therebyleaving large voids in the preform. These difficulties are overcome bythe present invention.

The present process comprises depositing boron nitride coating onfilaments, forming substantially uniaxially aligned preforms of thecoated filaments, tape casting of the matrix-forming material,laminating the coated filament preforms between ceramic tapes, binderburnout of the laminated structure, and hot-pressing for consolidation.

Those skilled in the art will gain a further and better understanding ofthe present invention from the detailed description set forth below,considered in conjunction with the accompanying figures which form apart of the specification wherein:

FIG. 1 shows a cross-section (perpendicular to the axis of the layers ofboron nitride coated filaments) of the present composite;

FIG. 2 shows a graph (continuous line) illustrating the load deflectionbehavior of the present composite having a mullite matrix, and anothergraph (broken line) illustrating the load deflection behavior of amonolithic hot pressed body of mullite; and

FIG. 3 shows a graph (continuous line) illustrating the load-deflectionbehavior of the present composite having a zircon matrix, and anothergraph (broken line) illustrating the load deflection behavior of amonolithic hot pressed body of zircon.

Briefly stated, the present process for producing a composite containinga layer of spaced, substantially parallel boron nitride coated filamentsembedded in a ceramic matrix comprises:

(a) providing matrix-forming ceramic material;

(b) admixing said matrix-forming material with an organic bindingmaterial;

(c) forming the resulting mixture into a tape;

(d) depositing a coating of boron nitride on a filament leaving nosignificant portion thereof exposed, said filament having a diameter ofat least about 50 microns and a length at least about 10 times itsdiameter;

(e) forming a plurality of said boron nitride coated filaments into alayer wherein said coated filaments are spaced from each other and atleast substantially parallel to each other;

(f) disposing said layer of coated filaments between the faces of two ofsaid tapes forming a layered structure;

(g) laminating the layered structure to form a laminated structure;

(h) heating said laminated structure to remove said organic bindingmaterial leaving no significant deleterious residue; and

(i) hot pressing the resulting porous structure at a sufficienttemperature under a sufficient pressure for a sufficient period of timeto consolidate said structure to produce said composite having aporosity of less than about 5% by volume, said composite containing nosignificant amount of reaction product of said coated filaments and saidmatrix, said matrix having a thermal expansion coefficient which rangesfrom lower than that of said coated filaments to less than about 15%higher than that of said coated filaments, at least about 10% by volumeof said composite being comprised of spaced coated filaments.

The present filament has a diameter of at least about 50 microns.Generally, the diameter of the filament ranges from about 50 microns toabout 250 microns, frequently from about 70 microns to about 200microns, or from about 100 microns to about 150 microns. The filament iscontinuous and can be as long as desired. It has a minimum length of atleast about 10 times its diameter, and generally, it is longer thanabout 1000 microns, or it is longer than about 2000 microns.

The minimum diameter of the present filament depends largely on theminimum spacing required between the boron nitride coated filamentsthrough which the matrix forming material must penetrate and isdeterminable empirically. For a given volume fraction of coatedfilaments, as the diameter of the coated filament decreases, the totalamount of space between coated filaments decreases making it moredifficult for the matrix-forming ceramic to penetrate the space. As aresult, the present invention enables the production of a composite witha high volume fraction of uniaxially aligned spaced continuous coatedfilaments not attainable with filaments of smaller diameter. Generally,filaments having a diameter of less than about 50 microns are not usefulin the present invention because they may not be practical to use orthey may be inoperable for producing composites with a desired highvolume fraction of filaments.

Preferably, the present filament has in air at ambient or roomtemperature, i.e. from about 20° C. to about 30° C., a minimum tensilestrength of about 100,000 psi and a minimum tensile modulus of about 25million psi.

In the present invention, the filaments can be amorphous, crystalline ora mixture thereof. The crystalline filaments can be single crystaland/or polycrystalline. Preferably, the filament is selected from thegroup consisting of aluminum oxide, elemental carbon, a siliconcarbide-containing material, and a silicon nitride-containing material.The silicon carbide-containing material contains at least about 50% byweight of silicon and at least about 25% by weight of carbon, based onthe weight of the material. Examples of silicon carbide-containingmaterials are silicon carbide, Si-C-O, Si-C-O-N, Si-C-O-Metal andSi-C-O-N-Metal where the Metal component can vary but frequently is Tior Zr. The silicon nitride-containing material contains at least about50% by weight of silicon and at least about 25% by weight of nitrogen,based on the weight of the silicon nitride-containing material. Examplesof silicon nitride-containing materials are silicon nitride, Si-N-O,Si-C-O-N, Si-N-O-Metal and Si-C-O-N-Metal where the metal component canvary but frequently is Ti or Zr. There are processes known in the artwhich use organic precursors to produce filaments which may introduce awide variety of elements into the filaments.

As used herein, filaments of "elemental carbon" or "carbon" includes allforms of elemental carbon including graphite.

Reference herein to filaments of silicon carbide includes, among others,presently available materials wherein silicon carbide material envelopsa core, and which generally are produced by chemical vapor deposition ofsilicon carbide on a core such as, for example, elemental carbon ortungsten.

In carrying out the present process, boron nitride is coated on thefilament to produce a coating thereon which leaves at least nosignificant portion of the filament exposed, and preferably, the entirefilament is coated with boron nitride. Preferably the entire wall ofeach individual filament is totally coated with boron nitride leavingnone of the wall exposed. The ends of the filament may be exposed butsuch exposure is not considered significant. The boron nitride coatingshould be continuous, free of any significant porosity and preferably itis pore-free. Preferably, the boron nitride coating is of uniform or atleast significantly uniform thickness.

The boron nitride coating can be deposited on the filament by a numberof known techniques under conditions which have no significantdeleterious effect on the filament. Generally, the boron nitride coatingcan be deposited by chemical vapor deposition by reactions such as:

    B.sub.3 N.sub.3 H.sub.6 (g)→3BN(s)+3H.sub.2 (g)     (1)

    B.sub.3 N.sub.3 H.sub.3 Cl.sub.3 (g)→3BN(s)+3HCl(g) (2)

    BCl.sub.3 (g)+NH.sub.3 (g)→BN(s)+3HCl(g)            (3)

Generally, the chemical vapor deposition of boron nitride is carried outat temperatures ranging from about 900° C. to 1800° C. in a partialvacuum, with the particular processing conditions being known in the artor determinable empirically.

The boron nitride coating should be at least sufficiently thick to becontinuous and free of porosity, or free of significant porosity.However, the coating is sufficiently thin so that the thermal expansioncoefficient of the boron nitride coated filament is the same as, or notsignificantly different from, that of the uncoated filament. Generally,the thickness of the coating ranges from about 0.3 microns to about 5microns, and typically it is about 0.5 microns. The particular thicknessof the coating is determinable empirically, i.e. it should be sufficientto prevent reaction, or prevent significant reaction, between thefilament and the matrix-forming material under the particular processingconditions used or under the particular conditions of use of theresulting composite. In the present invention, the boron nitride coatingbars contact, or bars significant contact, between the filaments and thematrix-forming material or matrix.

In one embodiment, the matrix-forming material is comprised of ceramicparticulates. These particulates are inorganic, polycrystalline, and inthe present process, they are consolidated, i.e. they undergo solidstate sintering, to produce the present solid composite. Thematrix-forming particulates can be comprised of an oxide-based ceramicsuch as, for example, aluminum oxide, mullite or zircon. Other suitablematrix-forming materials are silicon carbide and silicon nitride. Theparticulates are of a size which can penetrate between the filaments.Generally, they have a specific surface area ranging from about 0.2 toabout 10 meters² per gram, and frequently, ranging from about 2 to about4 meters² per gram.

In another embodiment of the present invention, the matrix-formingmaterial is comprised of a mixture of the ceramic particulates andwhiskers to produce a matrix with significantly increased toughness. Thewhiskers are crystalline, inorganic and stable in the present process.Preferably, the whiskers are comprised of silicon carbide or siliconnitride. The whiskers are of a size which can penetrate between thefilaments. Generally, they are less than about 50 microns in length andless than about 10 microns in diameter. Generally, the whiskers range upto about 30% by volume, frequently up to about 10% by volume, of thematrix-forming material.

In the present invention, the matrix-forming material, or matrix in thecomposite, has a thermal expansion coefficient ranging from lower thanthat of the coated filaments to less than about 15% higher than that ofthe coated filaments. Depending on such factors as filament size,alignment of the filaments and the particular processing conditions, amatrix-forming material with a thermal expansion coefficient about 15%or more higher than that of the coated filaments may result in a matrixwith significantly deleterious cracks which would render the compositeuseless. Preferably, for optimum mechanical properties of the composite,the matrix-forming material, or matrix, has a thermal expansioncoefficient ranging from less than to about the same as that of thecoated filaments.

In the present process, the components forming the composite, i.e. boronnitride coated filaments and matrix-forming material, are solid. Also,there is no significant amount of reaction product formed, or noreaction product detectable by scanning electron microscopy, between thecomponents of the matrix-forming material, or between the matrix-formingmaterial, or matrix, and coated filaments.

The organic binding material used in the present process bonds theparticulates, or particulates and whiskers, together and enablesformation of the required thin tape of desired solids content. By solidscontent, it is meant herein the content of matrix-forming material. Theorganic binding material thermally decomposes at an elevated temperatureranging to below about 800° C., generally from about 50° C. to belowabout 800° C., and preferably from about 100° C., to about 500° C., togaseous product of decomposition which vaporizes away and may leave aminor amount, not significant, of elemental carbon.

The organic binding material is a thermoplastic material with acomposition which can vary widely and which is well known in the art orcan be determined empirically. Besides an organic polymeric binder itcan include an organic plasticizer therefor to impart flexibility. Theamount of plasticizer can vary widely depending largely on theparticular binder used and the flexibility desired, but typically, itranges up to about 50% by weight of the total organic content.Preferably the organic binding material is soluble in a volatilesolvent.

Representative of useful organic binders are polyvinyl acetates,polyamides, polyvinyl acrylates, polymethacrylates, polyvinyl alcohols,polyvinyl butyrals, and polystyrenes. The useful molecular weight of thebinder is known in the art or can be determined empirically. Ordinarily,the organic binder has an average molecular weight at least sufficientto make it retain its shape at room temperature and generally such anaverage molecular weight ranges from about 20,000 to about 200,000,frequently from about 30,000 to about 100,000.

Representative of useful plasticizers are dioctyl phthalate, dibutylphthalate, diisodecyl glutarate, polyethylene glycol and glyceroltrioleate.

In carrying out the present process, the matrix-forming material andorganic binding material are admixed to form a uniform or at least asubstantially uniform mixture or suspension which is formed into a tapeof desired thickness and solids content. A number of conventionaltechniques can be used to form the mixture and resulting green tape.Generally, the components are milled in an organic solvent in which theorganic material is soluble or at least partially soluble to produce acastable mixture or suspension. Examples of suitable solvents are methylethyl ketone, toluene and alcohol. The mixture or suspension is thencast into a tape of desired thickness in a conventional manner, usuallyby doctor blading which is a controlled spreading of the mixture orsuspension on a carrier from which it can be easily released such asTeflon. The cast tape is dried to evaporate the solvent therefrom toproduce the present tape which is then removed from the carrier.

The particular amount of organic binding material used in forming themixture is determinable empirically and depends largely on the amountand distribution of solids desired in the resulting tape. Generally, theorganic binding material ranges from about 25% by volume to about 50% byvolume of the solids content of the tape.

The present tape, i.e. sheet, can be as long and as wide as desired, andgenerally it is of uniform or substantially uniform thickness. Itsthickness depends largely on the volume fraction of coated filamentswhich must be accommodated and is determinable empirically. The tapeshould be at least sufficiently thick to contain an amount ofmatrix-forming material required in the present process to produce thecomposite. Generally, with increasing volume fractions of coatedfilaments, correspondingly smaller amounts of matrix-forming materialwould be required. Generally, the tape has a thickness ranging fromabout 25 microns (0.001 inch) to about 1300 microns (0.052 inch),frequently ranging from about 125 microns (0.005 inch) to about 1000microns (0.040 inch), and more frequently ranging from about 250 microns(0.01 inch) to about 500 microns (0.02 inch).

Generally, in carrying out the present process, a preform comprised of alayer of the boron nitride coated filaments which are spaced from eachother and which are parallel, or at least substantially parallel, toeach other is used. The minimum space between the coated filamentsshould be at least sufficient to enable the matrix-forming material topenetrate therebetween, and generally, it is at least about 50 microns,and frequently at least about 100 microns. Generally, the spacingbetween coated filaments in a single layer is substantially equivalent,or if desired, it can vary. Coated filament loading in the composite canbe varied by changing the spacing between the coated filaments and/ortape thickness.

The preform of coated filaments can be produced by a number ofconventional techniques. For example, the coated filaments can beuniaxially aligned and spaced by placing them in a suitable deviceprovided with grooves and the desired spacing. The layer of coatedfilaments can be lifted off the device with adhesive tape placed acrossboth ends of the filaments. The taped end portions of the coatedfilaments can eventually be cut away from the laminated structure.

A layer of boron nitride coated filaments is placed between, i.e.intermediate, two tapes, i.e. between the faces of the tapes, to form alayered structure, i.e. substantially a sandwich structure. A pluralityof layers of coated filaments can be used in forming the layeredstructure provided they are separated from each other by a tape.Preferably, all of the tapes in the layered structure are at leastsubstantially coextensive with each other.

In one embodiment, before assembly of the layered structure, a solutionof the present organic binder in organic solvent is sprayed on the facesof the tapes to be contacted with the coated filaments, dried toevaporate the solvent and leave a sticky film of organic binder toenhance adhesion. The concentration of organic binder in solution canvary widely and generally ranges from about 1% by weight to about 10% byweight of the solution. The solution is sprayed on the face of the tapefor a period of time, determinable empirically, so that on evaporationof the solvent sufficient sticky binder remains to significantly enhanceadhesion or facilitate bonding of the tapes. Preferably, drying iscarried out in air at ambient temperature in less than a minute, andtypically, in a few seconds. The deposited binder can be a continuous ordiscontinuous coating, and typically, 0.2 milligrams of sticky binderper square centimeter of surface is adequate.

The layered structure is then laminated under a pressure and temperaturedeterminable empirically depending largely on the particular compositionof the organic binding material to form a laminated structure.Lamination can be carried out in a conventional manner. Laminatingtemperature should be below the temperature at which there isdecomposition, or significant decomposition, of organic binding materialand generally, an elevated temperature below 150° C. is useful and thereis no significant advantage in using higher temperatures. Typically, thelamination temperature ranges from about 35° C. to about 95° C. and thepressure ranges from about 500 psi to about 3000 psi. Generally,lamination time ranges from about 1/2 to about 5 minutes. Also,generally, lamination is carried out in air.

If desired, the laminated structure can be cut to desired dimensions bysuitable means such as a diamond saw.

The laminated structure is heated to thermally decompose the organicbinding material producing a porous structure comprised of the coatedfilaments and matrix-forming material. The rate of heating dependslargely on the thickness of the sample and on furnace characteristics.At a firing temperature ranging up to about 500° C., a slower heatingrate is desirable because of the larger amounts of gas generated atthese temperatures by the decomposition of the organic binding material.Typically, the heating rate for a sample of less than about 6millimeters (6000 microns) in thickness can range from about 15° C. perhour to about 30° C. per hour. At a temperature of less than about 800°C., thermal decomposition is completed leaving no significantdeleterious residue. Frequently, thermal decomposition of the organicbinding material leaves a residue of elemental carbon in thematrix-forming material generally ranging from an amount detectable byscanning electron microscopy to less than about 0.1% by volume, or lessthan about 0.05% by volume, of the total volume of the resulting porousstructure.

Thermal decomposition can be carried out in any atmosphere, preferablyat or below atmospheric pressure, which has no significant deleteriouseffect on the sample such as, for example, argon. Preferably, thermaldecomposition is carried out in a partial vacuum to aid in removal ofgases.

The resulting porous structure is hot pressed at a sufficienttemperature under a sufficient pressure for a sufficient period of timeto consolidate the structure to produce the present composite. Theparticular pressure, temperature and time are determinable empiricallyand are interdependent. Hot pressing temperature can vary dependinglargely on the characteristics of the matrix-forming material, theapplied pressure and hot pressing time. Generally under higher appliedpressures and longer times, lower hot pressing temperatures can be used.Likewise, under lower applied pressures and shorter times, higher hotpressing temperatures would be used. Generally, the hot pressingtemperature is at least about 1400° C., generally ranging from about1400° C. to about 1700° C., frequently from about 1500° C. to about1650° C., and more frequently from about 1550° C. to about 1600° C.Generally, temperatures below about 1400° C. are likely to produce acomposite having a porosity greater than about 5% whereas temperaturesabove about 1700° C. may coarsen the grains in the product and noteffect density.

Generally, hot pressing pressure ranges from higher than about 100 psito a maximum pressure which is limited by the creep of the sample, i.e.there should be no significant deformation by creep of the sample.Frequently, hot pressing pressure ranges from about 1000 psi or about2000 psi to about 8000 psi. It is advantageous to use a pressure closeto the maximum available because the application of such high pressuremakes it possible to keep the pressing temperature low enough to controlgrain growth. Generally, hot pressing is carried out in a period of timeranging up to about 30 minutes and longer periods of time usually do notprovide any significant advantage. Generally, after hot pressing, thesample is furnace cooled to about room temperature before beingrecovered from the press.

Hot pressing is carried out in a non-oxidizing atmosphere. Moreparticularly, it is carried out in a protective atmosphere in which thematerial is substantially inert, i.e. an atmosphere which has nosignificant deleterious effect thereon. Representative of the hotpressing atmospheres is nitrogen, argon, helium or a vacuum. The hotpressing atmosphere generally can range from a substantial vacuum toabout atmospheric pressure.

In the present process, there is no loss, or no significant loss, of thecomponents forming the present composite, i.e. boron nitride coatedfilaments and matrix-forming material.

The present composite is comprised of spaced boron nitride coatedfilaments embedded in and/or within a matrix. The coated filaments canbe enveloped by matrix with or without their ends or end portionsexposed. The matrix is continuous and interconnecting. It is distributedthrough the coated filaments and generally it is space filling orsubstantially completely space filling. Generally, the matrix envelopsor surrounds each boron nitride coated filament, or each coated filamentof more than 99% by volume of the coated filaments, embedded therein.The matrix has a thermal expansion coefficient which ranges from lowerthan that of the coated filaments to less than about 15% higher thanthat of the coated filaments. Preferably, the matrix has a thermalexpansion coefficient which is about the same as, or which is lowerthan, the thermal expansion coefficient of the coated filaments.

The composite may contain a single layer of coated filaments or aplurality of layers of coated filaments. In a composite containing aplurality of layers of coated filaments, there is no contact between thelayers and they are separated by matrix material. In each layer, morethan 99% by volume of the coated filaments, and preferably all orsubstantially all of the coated filaments, are spaced from each otherand parallel or at least substantially parallel, to each other. Morethan 99% by volume, or all, of the coated filaments in each layer arealigned, or substantially aligned, in a single plane. Any misalignmentof the coated filaments should not significantly degrade the mechanicalproperties of the composite.

The spaced coated filaments in a layer or plurality of layers compriseat least about 10% by volume of the composite. Generally, the coatedfilaments range from about 10% by volume to about 70% by volume,frequently from about 20% by volume to about 60% by volume, or fromabout 30% by volume to about 50% by volume, of the composite.

The filaments in the composite are coated with boron nitride which is atleast detectable by scanning electron microscopy and generally ranges inthickness from about 0.5 microns to about 1.5 microns. The particularamount of boron nitride in the composite provided by the boron nitridecoating depends largely on the amount of coated filaments present, thethickness of the boron nitride coating and the diameter of the filament.Therefore, the volume fraction of boron nitride provided by the coatingis the balance of the volume fraction of all other components of thecomposite. Frequently, however, the boron nitride coating on thefilaments in the composite generally ranges from less than about 1% byvolume to about 20% by volume, or from about 1% by volume to about 10%by volume, or from about 1% by volume to about 5% by volume, of thetotal volume of boron nitride-coated filaments. The boron nitridecoating can be amorphous, crystalline, or a combination thereof.

The present boron nitride coating optimizes interfacial shear stressbetween the filaments and matrix resulting in a composite with atoughness significantly or substantially higher than that of a compositewherein the filaments are uncoated. Specifically, if the matrix andfilaments were in direct contact, even a slight reaction therebetweenwould increase interfacial bonding thereby requiring a higher stress topull out the filaments making the composite less tough. If theinterfacial bonding were too high, then the composite would fail in abrittle manner. In contrast, the present boron nitride coating providesan interfacial shear stress which is significantly lower than thatproduced with uncoated filaments thereby allowing the coated filamentsto pull out more easily and gives the composite more toughness.

The matrix is comprised of a solid state sintered ceramic, i.e.polycrystalline, phase. In another embodiment, the matrix is comprisedof a mixture of the sintered phase and a phase of whiskers. Preferably,the sintered ceramic phase has an average grain size of less than about100 microns, or less than about 50 microns, or less than about 20microns, and most preferably less than about 10 microns.

In one embodiment, the matrix of the composite is free of elementalcarbon which is detectable by scanning electron microscopy. In anotherembodiment, the matrix contains elemental carbon ranging from an amountdetectable by scanning electron microscopy to less than about 0.1% byvolume, or less than about 0.05% by volume, of the composite. Anyelemental carbon present in the matrix has no significant deleteriouseffect on the composite.

The boron nitride coated filaments impart significant toughness andprevent brittle fracture of the composite at room temperature. Bybrittle fracture of a composite it is meant herein that the entirecomposite cracks apart at the plane of fracture. In contrast to abrittle fracture, the present composite exhibits filament pull-out onfracture at room temperature. Specifically, as the present compositecracks open, generally at least about 10% by volume, frequently at leastabout 30% or 50% by volume, of the coated filaments, and preferably allof the coated filaments, pull out and do not break at the plane offracture at room temperature.

One particular advantage of this invention is that the present compositecan be produced directly in a wide range of sizes. For example, it canbe as long or as thick as desired.

The present composite has a porosity of less than about 5% by volume,preferably less than about 1% by volume, of the composite. Mostpreferably, the composite is void- or pore-free, or has no significantporosity, or has no porosity detectable by scanning electron microscopy.Generally, any voids or pores in the composites are less than about 70microns, preferably less than about 50 microns or less than about 10microns, and they are distributed in the composite. Specifically, anyvoids or pores are sufficiently uniformly distributed throughout thecomposite so that they have no significant deleterious effect on itsmechanical properties.

The present composite has a wide range of applications depending largelyon its particular composition. For example, it is useful as a wearresistant part, acoustical part or high-temperature structuralcomponent.

As a practical matter, the present composite cannot be produced by hotpressing the coated filaments between layers of loose ceramic particlesbecause during hot pressing the particles slide pushing the filamentsout of alignment, and frequently, also into contact with each other.

The invention is further illustrated by the following examples where,unless otherwise stated, the procedure was as follows:

Commercially available continuous filaments of silicon carbide producedby a chemical vapor deposition process and sold under the trademark AVCOSCS-6 were used. These filaments had a 35 micron carbon core on whichsilicon carbide was deposited to an overall diameter of about 145microns. The outside surface of the filaments consisted of two layers ofpyrolytic carbon and carbon-silicon, with overall thickness of about 3microns. In air at room temperature these filaments have a tensilestrength of about 500 thousand psi and a tensile modulus of about 60million psi. These filaments have an average thermal expansioncoefficient of less than about 5.0×10⁻⁶ in/in-°C.

The filaments were cut to a length of about 2 inches and were coatedwith boron nitride by the following low pressure chemical vapordeposition process utilizing the reaction B₃ N₃ H₃ Cl₃ →3BN+3HCl.Specifically, the filaments were placed on a molybdenum screen which wasthen positioned at about the midpoint of the hot zone of apyrex/quartz/pyrex furnace tube. A 1.00 gram sample of commercialtrichloroborazine (B₃ N₃ H₃ Cl₃) was transferred in an argon-filledglove box to a pyrex end-section which contained a thermocouple vacuumgauge, a cold trap and a vacuum stopcock. The closed pyrex end-sectionwas then taken out of the glove box and attached to an end of thefurnace tube and to a vacuum system. The end-section containing thetrichloroborazine was then cooled using liquid nitrogen and the furnacetube was opened to the vacuum system via the stopcock of the pyrexend-section. After the system reached a pressure lower than 0.020 torr,the furnace was heated to about 1050° C. When the pressure had againdropped below 0.020 torr and the furnace temperature had stabilized, theend-section containing the trichloroborazine was warmed by an oil bathmaintained at 60° C., whereupon the solid began to vaporize, depositingBN and liberating gaseous HCl in the hot zone of the furnace tube andproducing an increase in pressure. The pressure was observed to reach ashigh as about 200 torr before stabilizing at about 50 torr. After twohours, the pressure was found to have decreased to about 0.020 torr,whereupon the furnace was shut down and the system allowed to cool toroom temperature before opening the tube and removing the sample.Identification of the chemically vapor deposited layer as BN wasaccomplished by means of electrical resistance measurement and aquantitative ESCA analysis of a film deposited in substantially the samemanner on a SiC disk surface. This film was amorphous to x-rays in theas-deposited condition and appeared fully dense and smooth at highmagnification in the SEM. Scanning electron microscopy observation ofthe ends of coated and broken filaments revealed that the coating wascontinuous and smooth and about 1.5 microns thick on the filament andleft no significant portion of the filament exposed.

The boron nitride coated filaments were uniaxially aligned by placingthem in a device for aligning filaments and maintaining the requiredspacing between them. This device was made from a copper foil laminatedon a printed circuit board which was etched by the photolithographictechnique in such a way as to produce parallel grooves about 0.06 inchdiameter, 0.0004 inch deep, and 0.0008 inch apart (center-to-center).The coated filaments were placed on this device and a simple scoop ofthe filaments using a straight edge led to filling of each of thegrooves with a filament. This resulted in a single layer of uniformlyspaced coated filaments which was lifted off the board by puttingadhesive tapes across each end portion of the filament layer. Theadhesive tapes were sufficient to maintain the alignment and spacingbetween the coated filaments in the layer. Several such pre-formedlayers of coated filaments were produced in which the coated filamentswere substantially parallel and spaced about 100 microns from eachother.

The mullite powder had an average size of about 0.7 microns.

The zircon powder had an average size of about 0.5 microns.

By ambient temperature herein it is meant room temperature, i.e. fromabout 20° C. to about 30° C.

The organic binding material was comprised of commercially availableorganic binder comprised of polyvinylbutyral (average molecular weightof about 32,000) and commercially available liquid plasticizer comprisedof polyunsaturated hydroxylated low-molecular weight organic polymers.Specifically, in Examples 1 and 2, the organic binding material wascomprised of 4.86 grams of polyvinylbutyral and 9.4 grams of liquidplasticizer, and in Examples 3 and 4, it was comprised of 4.44 grams ofpolyvinylbutyral and 4.01 grams of liquid plasticizer.

Hot pressing was carried out in a 2 inch inner diameter 2 inch innerlength cylindrical die in an atmosphere of flowing nitrogen which was atabout atmospheric pressure.

Standard techniques were used to characterize the hot pressed compositefor density, microstructure and mechanical properties.

EXAMPLE 1

Mullite tapes were prepared by the tape casting technique. 14.26 gramsof the organic binding material were dissolved at ambient temperature in44 grams of a mixture of 34 grams of toluene and 10 grams of methylisobutyl ketone. The resulting solution was admixed with 100 grams ofmullite powder in a ball mill for about 4 hours at ambient temperature.The resulting slurry was tape cast on a Mylar sheet using a doctorblade, then dried in air at room temperature and atmospheric pressure toremove the solvent, and the resulting tape was stripped from the Mylarsheet.

The tape was about 6 inches wide and had a substantially uniformthickness of about 0.012 inch. Mullite powder was distributed thereinsubstantially uniformly.

The tape was cut to lengths of about 1.5 inches. Each of the resultingtapes contained mullite powder in an amount of about 52% by volume ofthe tape.

A layered sandwich-type structure was formed comprised of 7 layers ofmullite tapes and 6 layers of boron nitride coated filaments wherein thecoated filament layers were separated from each other by tape. Theadhesive-taped portions of the coated filaments protruded from thelayered structure. Before assembly, to enhance adherence, the faces ofthe tapes which were to be contacted with the coated filaments weresprayed with an organic solution of binder, dried for a few seconds inair at room temperature leaving a discontinuous coating of stickyorganic binder. Specifically, a solution comprised of 3 weight % ofcommercially available polyvinylbutyral (average molecular weight ofabout 32000), 39 weight % toluene, 9.5 weight % acetone, 39 weight %xylene and 9.5 weight % ethanol was used. The solution was sprayed onthe faces of the tapes for a sufficient time so that upon evaporation ofthe solvent there remained about 0.2 milligrams of sticky organic binderper square centimeter of surface.

The resulting layered structure was laminated in air in a laminatingpress at about 93° C. under a pressure of about 1000 psi for about 1minute. At lamination temperature and pressure, the tapes were plasticresulting in filling of the void space between and around the coatedfilaments.

The laminated structure was sliced perpendicular to the filament axisinto bar-shaped samples (1.25 inch×0.3 inch×0.15 inch) using a diamondsaw. Examination of a cross-section showed uniform spacing between thecoated filaments as well as between the layers of filaments.

The samples were placed in a vacuum oven for removing the organicbinding material wherein the vacuum was typically about 20 millitorr.The burnout cycle was comprised of heating the furnace at a rate of 30°C. per hour to 500° C., a five hour hold at 500° C. and a cooldown toroom temperature at a rate of 200° C. per hour. This led to completeremoval of the organic matter from the laminated structure whichresulted in a porous structure comprised of mullite powder andfilaments. No elemental carbon was detected by scanning electronmicroscopy.

Each of the porous bar-shaped structures was placed in a graphite dieand hot-pressed at temperatures ranging from about 1570° C. to about1585° C. Each sample was heated at a rate of approximately 100° C. perminute to the maximum hot pressing temperature under a pressure of 3500psi applied for consolidation. The consolidation was monitored byplunger displacement and complete densification occurred within 30minutes after the onset of densification. After hot-pressing, the samplewas furnace cooled to room temperature and removed from the die.

The hot pressed samples, i.e. composites, were characterized and areillustrated in Table I.

The cross section (perpendicular to filament axis) of one of thecomposites (Example 1C) is shown in FIG. 1. FIG. 1 illustrates thesubstantially uniform spacing between the coated filaments as well asthe substantially uniform spacing between layers of coated filaments.FIG. 1 also shows that each layer of coated filaments was maintained ina substantially single plane. In addition, FIG. 1 shows a fully densemullite matrix surrounding each individual filament. The density of thiscomposite was 3.10 g/cc, in line with fully dense mullite matrixmaterial containing about 25 volume % filaments. No porosity wasdetected in the composite by microscopy. Mullite has an average thermalexpansion coefficient of about 5.0×10⁻⁶ in/in-°C. and less than about15% higher than that of the coated filaments.

Some of the composites were broken at room temperature in athree-point-bend configuration to determine fracture strength andload-elongation characteristics. All of the broken composites exhibitedfilament pullout, i.e. more than 10% by volume of the filaments pulledout and did not break at the plane of fracture. The results for each ofthree of the composites are given in Table I as Examples 1A-1C.

FIG. 2 shows a load deflection curve for the composite of Example 1C. Itcan be seen that this composite showed toughened ceramic-like behavior.The load-deflection curve shows the onset of matrix cracking followed bya significant rise in the load carrying capability of the composite. Anultimate load was reached beyond which the composite showed gracefulfailure i.e., gradual decrease in the load on extensive failure of thecomposite.

For comparison, mullite powder alone was hot pressed in substantiallythe same manner as the sample of Example 1B to produce a body (Example1X in Table I) of substantially the same size and density which wasfractured in substantially the same manner. Its load deflection curve(broken line) is also shown in FIG. 2. It fractured in a brittle manner.

EXAMPLE 2

This example was carried out in substantially the same manner as Example1 except as noted herein and in Table I.

The dried tape had a thickness of about 0.015 inch and contained avolume fraction of mullite powder of about 54% by volume of the tape.

A number of composites were produced and some were broken as disclosedin Example 1. All of the broken composites showed toughened ceramic-likebehavior and filament pullout, i.e. more than 10% by volume of thecoated filaments pulled out and did not break at the plane of fracture.The results for one of these composites is given as Example 2A in TableI.

EXAMPLE 3

This example was carried out in substantially the same manner as Example1 except as noted herein and in Table I. Zircon powder was used insteadof mullite powder.

100 grams of zircon powder and a solution of 8.4 grams of organicbinding material and 34.3 grams of a mixture of 26.4 grams toluene, 6.5grams methyl isobutyl ketone and 1.4 grams ethyl alcohol were used toform the slurry.

The dried tape had a thickness of about 0.010 inch and contained avolume fraction of zircon powder of about 53 % by volume of the tape.

A number of composites were produced and some were broken as disclosedin Example 1. All of the broken composites showed toughened ceramic-likebehavior and filament pullout, i.e. more than 10% by volume of thecoated filaments pulled out and did not break in the plane of thefracture. The results for each of three of the composites are given asExamples 3A-3C in Table I. Zircon has an average thermal expansioncoefficient of less than about 5.0×10⁻⁶ in/in-°C. and less than about15% higher than that of the coated filaments.

FIG. 3 shows a load deflection curve for the composite of Example 3A. Itcan be seen that this composite showed toughened ceramic-like behavior.The load-deflection curve shows the onset of matrix cracking followed bya significant rise in the load carrying capability of the composite. Anultimate strength was reached beyond which the composite showed gracefulfailure, i.e. gradual decrease in the load on extensive failure of thecomposite.

For comparison, zircon powder alone was hot pressed in substantially thesame manner as the sample of Example 3A to produce a body (Example 3X inTable I) of substantially the same size and density which was fracturedin substantially the same manner. Its load deflection curve (brokenline) is also shown in FIG. 3. It fractured in a brittle manner.

EXAMPLE 4

This example was carried out in substantially the same manner as Example3 except as noted herein and in Table I.

The dried tape had a thickness of about 0.012 inch and contained avolume fraction of zircon powder of about 53% by volume of the tape.

A number of composites were produced substantially as disclosed inExample 3 and some were broken as disclosed in Example 1. All of thebroken composites showed toughened ceramic-like behavior and filamentpullout, i.e. more than 10% by volume of the coated filaments pulled outand did not break in the plane of fracture. The results for one of thesecomposites is given as Example 4A in Table I.

                                      TABLE I                                     __________________________________________________________________________    Tape                                                                                  Vol. %   Filament    Composite Characteristics                           Matrix-                                                                            matrix-                                                                            Thick-                                                                            Layers in                                                                          Hot Pressing      Avg.                                                                              Fracture                                                                           Fracture                        forming                                                                            forming                                                                            ness                                                                              Layered                                                                            Temperature                                                                          Density                                                                            Filaments                                                                           grain                                                                             strength                                                                           strain                       Ex.                                                                              material                                                                           material                                                                           (inch)                                                                            Structure                                                                          (°C.)                                                                         g/cc Vol. %                                                                              size                                                                              (MPa)                                                                              (%)                          __________________________________________________________________________    1A mullite                                                                            52   0.012                                                                             6    1585   3.09 25    <5 μm                                                                          777  1.21                         1B "    52   0.012                                                                             6    1580   3.09 25    "   855  1.02                         1C "    52   0.012                                                                             6    1570   3.09 25    "   646  0.90                         1X "    52   0.012                                                                             none 1580   3.13  0    "   202  0.16                         2A "    54   0.015                                                                             6    1575   3.09 25    "   750  1.1                          3A zircon                                                                             53   0.010                                                                             6    1570   4.14 25    "   712  1.4                          3B "    53   0.010                                                                             6    1560   4.14 25    "   657  1.1                          3C "    53   0.010                                                                             6    1565   4.19 25    "   712  1.0                          3X "    53   0.010                                                                             none 1570   4.36  0    "   215  0.15                         4A "    53   0.012                                                                             6    1525   4.11 25    "   645  0.7                          __________________________________________________________________________

All of the composites produced in Table I, except Examples 1X and 3X,illustrate the present invention. The present composites are useful ashigh temperature structural material in aircraft engines.

What is claimed is:
 1. A process for producing a composite containing alayer of boron nitride coated filaments embedded in a ceramic matrixwhich comprises the following steps:(a) providing matrix-forming ceramicmaterial; (b) admixing said matrix-forming material with an organicbinding material; (c) forming the resulting mixture into a tape; (d)depositing a coating of boron nitride on a filament leaving nosignificant portion thereof exposed, said filament having a diameter ofat least about 50 microns and a length at least about 10 times itsdiameter; (e) forming a plurality of said boron nitride coated filamentsinto a layer wherein said coated filaments are spaced from each otherand at least substantially parallel to each other; (f) disposing saidlayer of coated filaments intermediate the faces of two of said tapesforming a layered structure; (g) laminating the layered structure toform a laminated structure; (h) heating said laminated structure toremove said organic binding material leaving no significant deleteriousresidue; and (i) hot pressing the resulting porous structure at asufficient temperature under a sufficient pressure for a sufficientperiod of time to consolidate said structure to produce said compositehaving a porosity of less than about 5% by volume, said compositecontaining no significant reaction product of said coated filaments andsaid matrix, said matrix having a thermal expansion coefficient whichranges from lower than that of said coated filaments to less than about15% higher than that of said coated filaments, at least about 10% byvolume of said composite being comprised of spaced boron nitride coatedfilaments.
 2. The process according to claim 1 wherein said filamentsare selected from the group consisting of aluminum oxide, elementalcarbon, a silicon carbide-containing material containing at least about50% by weight of silicon and at least about 25% by weight of carbonbased on the weight of said material, and a silicon nitride-containingmaterial containing at least about 50% by weight of silicon and at leastabout 25% by weight of nitrogen based on the weight of said siliconnitride-containing material.
 3. The process according to claim 1 whereinsaid matrix-forming material is comprised of polycrystalline ceramicparticulates.
 4. The process according to claim 1 wherein saidmatrix-forming material is comprised of particulates selected from thegroup consisting of aluminum oxide, mullite, zircon, silicon carbide andsilicon nitride.
 5. The process according to claim 1 wherein saidmatrix-forming material is comprised of polycrystalline ceramicparticulates and whiskers wherein said whiskers range up to about 30% byvolume of said matrix-forming material.
 6. The process according toclaim 1 wherein before said layered structure is assembled, an organicsolution of organic binder is sprayed on the faces of the tapes to becontacted with the coated filaments and dried to leave a sticky film oforganic binder in amount sufficient to at least significantly enhanceadhesion.
 7. The process according to claim 1 wherein said matrix has athermal expansion coefficient ranging from lower than that of saidcoated filaments to about the same as that of said coated filaments. 8.The process according to claim 1 wherein a plurality of said layers ofcoated filaments is used in forming the layered structure and whereinall of said layers of coated filaments are separated from each other bysaid tape.